Oxide films form on aluminum alloys when they are exposed to an atmosphere containing oxygen. Specifically, aluminum readily oxidizes in the presence of air (Eqn. (1)), or moisture (Eqns. (2 and 3)), rapidly forming a thin, strong protective oxide film on any exposed aluminum metal surface, including both liquid and solid surfaces.4Al+3O2→2Al2O3  (1)3H2O+2Al→Al2O3+3H2  (2)H2→2[H]melt  (3)
Because aluminum oxide is very stable thermodynamically, it is typically present in all aluminum alloys. Therefore, any furnace charge contains unavoidable amounts of alumina as a typical coating, constituting an exogenous inclusion source. During the mold filling of the casting process, additional aluminum oxides are formed when the free surface of the melt front contacts air and particularly when the liquid melt velocity produces turbulent flow. A distinction is often made between oxides pre-existing in the melting furnace, referred to as “old oxides,” and those created during mold filling, called “young oxides.” Campbell, J., Castings, Elsevier Butterworth-Heinemann, 2003; Q. G. Wang, C. J. Davidson, J. R. Griffiths, and P. N. Crepeau, “Oxide Films, Pores and The Fatigue Lives of Cast Aluminum Alloys”, Metall. Mater. Trans. vol. 37B (2006), pp. 887-895. For young oxides, the cause of entrainment has been described as “surface turbulence,” a reference to phenomena such as two or more flow fronts joining together (bifilms, flow marks, folds, and cold shuts), a contraction of the surface area of a liquid (with folding of the oxide surface), or the passage of a bubble through the liquid.
Young oxides are more detrimental to material properties than old oxides. Because of the lack of wetting between oxide films folded dry side to dry side in young oxides, the entrained oxide unfurls during solidification and acts like a void or crack in the solidifying aluminum casting. These cracks can not only be initial sites for pore formation, but also be frozen into the solid and can significantly decrease the tensile and fatigue strengths of the casting. The bi-films can also cause hot tearing. Entrained oxides are believed to increase melt viscosity, and hence reduce fluidity, and adversely affect the feeding of castings. Surface oxide skins can significantly increase the apparent surface tension of melts and increase the possibilities of forming cold shuts, flow marks, and misruns.
In many cases, such as high pressure die casting, turbulent flow of the aluminum melt readily occurs. The problem of entrapped young oxides can arise if the velocity of the liquid metal is sufficiently high at some point in the flow to fall back under gravity and entrap a portion of its own surface. This critical velocity is believed between 0.45 m/s and 0.5 m/s for Al, Mg, Ti, and Fe alloys. Campbell, J., Castings, Elsevier Butterworth-Heinemann, 2003. It is desirable to remain under this critical velocity to significantly reduce the number of oxides in the casting. However, in gravity casting processes, velocities can easily exceed 0.5 m/s in the pour cup or the downsprue. Oxides consequently form even before metal enters the casting or runner system. These oxides can be carried into the casting and have the same detrimental effect as oxides formed in the runners or the casting cavity. Low pressure casting processes offer improved control over fill speeds so oxide formation is therefore generally reduced. However, velocities in the sprue can still exceed 0.5 m/s resulting in formation of entrained oxides.
To minimize and eventually eliminate the oxides in the final cast aluminum products, it is desired to predict oxide defects in cast aluminum components to be able to develop an optimized gating/riser system, filtration, and fill profile.
Although there is a strong practical need to simulate and predict the size and volume of aluminum young oxides formed during a mold filling process, no reliable method or technique has yet been reported.